During our previous GM R29-54523 project period we studied the formation of ditryptophan crosslinks by first forming tryptophan dimer crosslinks and oxidizing them to ditryptophans. We mad two important findings relating to the conformation of ditryptophan- linked peptides: they stabilize gamma turns and they can bridge antiparallel beta sheets. These findings serve as the basis for exploration and exploitation of the effects of ditryptophans on peptide structure and the extension of our work to dityrosines. Collectively these are referred to as biaryl crosslinks. The focus of this proposal is to synthesize peptides which are irreversibly (relative to destruction of the peptide backbone) linked by ditryptophans and dityrosines and to study the effects of these biaryl crosslinks on peptide conformation., We will continue to use the Mannich dimerization and oxidation of tryptophan sidechains for the synthesis of peptides bridged by ditryptophans. In addition, we will develop the first efficient method for the synthesis of dityrosine crosslinked peptides. With access to biaryl linked peptides we will study the effects of the biaryl crosslink on secondary structure. Our specific aims for this proposal are: 1. For the first time, develop a practical method for synthesis of dityrosine-linked peptides. 2. Determine whether ditryptophans can enforce local beta structure in solution 3. Determine whether dityrosines can enforce local beta structure 4. Determine whether a crosslink in the Trp-Trp dyad can stabilize a cis amide bond 5. Compare the structure of gramicidin B with and without ditryptophan crosslink 6. Determine if a dityrosine or ditryptophan peptide dimer can disrupt a protein-protein interaction It is not the goal of this funding period to target a specific disease state? The intrinsic health significance of dityrosines, and possibly ditryptophans, is that they are found in human proteins. We seek to understand the effect of these crosslinks (beneficial or detrimental) on local peptide structure. A second and more practical health significance is that the single covalent bond of a ditryptophan or dityrosine crosslink might allow us to prepare a new class of peptide dimers which can interfere with pharmacologically important protein-protein interactions.